The present invention relates to a selfoscillating mixer circuit using a single-gate field effect transistor (FET). In particular, the invention relates to a mixer circuit which is best suited for frequency conversion of a signal of a superhigh frequency of SHF-band.
There has been hitherto known an oscillator circuit to operate at SHF-band which comprises as a fundamental component a so-called micro strip line constituted by conductors of various configurations disposed in a planar manner on an insulation substrate having a rear surface covered wholly with an electrical conductive material and a FET, as is shown in FIG. 1. As can be seen from this figure, the FET 1 has a source terminal 2 grounded and a gate terminal 3 thereof connected to a drain terminal 4 through lines 5a and 5b which constitutes a feedback path. Reference character 5c represents a DC component blocking capacitor. Thus, the lines 5a and 5b may be considered to be directly connected to each other for AC signal components. The signal appearing at the drain terminal 4 is fed back to the gate terminal 3 through the lines 5a and 5c. The phase of the signal fed back to the gate terminal 3 will vary in dependence on the frequency thereof. Since a positive feedback takes place for a signal of a predetermined frequency, oscillation will occur at such a predetermined frequency. Thus, by selecting the length of the lines 5a and 5b so that the positive feedback may be provided at a desired frequency, at which the oscillation is to occur, the oscillator circuit of the desired frequency can be obtained. In FIG. 1, means for applying a DC bias voltage to an FET is not shown. Since such DC bias voltage applying means is irrelevant to the essence of the invention, the description as well as illustration thereof will be omitted herein.
In view of the fact that the signals of the same phase on a line of a certain length do not always have a single frequency but may have an integral multiple of that frequency, there may arise such a situation in which the oscillator frequency differs from the desired frequency. However, since the frequency at which the oscillation may readily take place is determined by the characteristic of an FET as actually used, it is a simple matter in practice to have the oscillator circuit of a desired frequency by correspondingly selecting the type of an FET to be used. In order to more positively assure the oscillation only at the desired frequency, the feedback lines 5a and 5b may be realized in a form of a low-pass filter to block harmonic components.
The circuit shown in FIG. 1 is constructed as a self-oscillating mixer circuit by providing an input line for radio frequency signals connected to the gate terminal 3 and an output line 7a connected to the drain terminal 4 and incorporating therein a low-pass filter 7 adapted to pass therethrough an intermediate frquency signal. With such circuit arrangement, it is possible to derive the intermediate frequency (IF) signal from the low-pass filter which has a frequency corresponding to the difference between the frequency of the radio frequency signal input from the line 6 and the oscillator frequency. When an IF signal is selected to have a frequency of UHF-band or VHF-band, the low-pass filter 7 can be easily implemented by the micro strip line. Since the IF signal is output with a gain relative to the amplitude of the input radio frequency signal, there can be an advantage such that the implementation of circuits succeeding to the low-pass filter 7 is accomplished in a facilitated manner as compared with a diode mixer or the like, and the oscillation, frequency conversion as well as amplification can be attained simultaneously in a simplified circuit configuration.
The self-oscillating mixer circuit may be also implemented by exchanging the connections to the source 2 and the gate 3 of FET 1.
It is also known to connect a bypass circuit 9 for bypassing intermediate frequency signals to the input line 6 for the radio frequency signal, as is shown in FIG. 2. This bypass circuit 9 may be constituted by a quarter-wavelength (1/4-.lambda.) line 9a so as to be opened for a radio frequency signal and a capacitor 9b for coupling the end of the 1/4-.lambda. line 9a to the ground in respect of AC signals. The capacitor 9b functions to prevent a DC voltage for the gate bias from being grounded and a capacitance of the capacitor 9b is so selected as to provide a sufficiently low impedance at the intermediate frequency. Since the intermediate frequency is set to be sufficiently low as compared with the radio frequency signals, the length of the line 9a and a portion of the line 6 extending from the gate terminal 3 to the capacitor 9b may be left out of consideration for the intermediate frequency signals. The gate terminal 3 may be regarded as being grounded. Thus, the intermediate frequency signals can be outputted from the drain terminal 4 with a high efficiency.
The self-oscillating mixer circuit of the arrangement described above has however a serious drawback that a remarkable leakage of the local oscillation signal into the radio frequency signal input circuit will occur through the radio frequency signal input line 6. With an attempt to evade such difficulty, it is contemplated to connect to the input line a trap circuit 8 for the local oscillation signal constituted by an open-ended line of a length corresponding to a quarter-wavelength of the oscillator-frequency signal, as is shown in FIG. 3. However, such a trap circuit 8 has a low Q and involves a loss to the radio frequency signal as well as damping of the input signal applied to the gate terminal 3, which is further aggravated by the fact that the local oscillator frequency is very close to that of the radio frequency signal. Further, since the oscillator frequency is determined by the phase shift of the signal due to the length of the lines 5a and 5b as described hereinbefore, the low Q of this feedback path exerts an adverse influence to the stability of the oscillator frequency.